For most Americans, as the nation's thoughts turn to the 10th anniversary of the 9/11 terrorist attacks, memories of that day readily come flooding back. People can remember with great clarity what they were doing or to whom they were speaking when they learned of the crisis—whether via a sister's phone call or a first-hand glimpse of the World Trade Center on fire.

How much can we trust, then, of what we remember of 9/11? Some answers are provided by a national study of 9/11 memories conducted by researchers at intervals of one week, one year and three years after the attack. The team surveyed more than 3,000 people in New York City, Washington, D.C., Boston, and four other cities in Connecticut, Missouri and California. Last month, the scientists did a 10-year follow-up survey—data yet to be analyzed—making the project the longest prospective study of how flashbulb memories change over time. Scientific American spoke with New York University psychologist Elizabeth A. Phelps, a lead investigator of the survey.

[An edited transcript of the interview follows.]

New York University (NYU) is only two miles from the World Trade Center site. What was it like when you got to your lab the morning of 9/11?
By then, the second plane had hit the towers. There was nobody else in the office at that point, but one person. I went to his office, which looked out at the World Trade Center, and we saw one tower go down. You just couldn't even believe it. And then it was kind of a weird day. Some people came into the office. You really couldn't focus on work. So eventually I went home, watched CNN and ended up trying to go give blood.

How did the 9/11 memory project come about?
Phone service was hit or miss that day, but John Gabrieli, a friend and neuroscientist who was then at Stanford, managed to call me. He just wanted to make sure I was okay. We started talking about doing a study of 9/11, because every memory researcher knows about the classic studies of memories of the Challenger space shuttle explosion or the 1989 San Francisco earthquake. Then after a day or so, we decided to do it.

We worked very fast. John started putting the survey questionnaire together with Kevin Ochsner, a postdoc in his group. We got in touch with Bill Hirst, Marcia Johnson, Chandan Vaidya and other researchers who joined the consortium. By September 18, we had survey tables in front of the NYU library and in Washington Square, and Bill [Hirst] had tables up around 14th Street, at the New School for Social Research. Right after 9/11, people in New York really wanted to be around other people and to do things that were helpful. And I think taking our survey was something that people thought would contribute to the greater good.

My entire lab worked on the 9/11 project. At the time, it would have been hard for us to work on anything else. The streets were closed around NYU and the campus shut down. The study helped us stay busy and yet feel relevant to what was going on.

How does emotion in a traumatic event like 9/11 influence our memories?
Emotion kind of focuses you on a few details but lets you ignore other details. And if you are highly aroused by fear, that emotion helps you store things in your memory better, in a storage process called consolidation that depends on the interaction of the amygdala and hippocampus. But what we've known for a while is that emotion gives you a stronger confidence in your memory than it does necessarily in the accuracy. Usually, when a memory has highly vivid details and you're confident in those details, that means you're likely to be right. Confidence often goes hand in hand with accuracy. But when something is highly emotional, they often get separated.

So if you look at memory for 9/11, pretty much everybody would say, "I know where I was, who I was with," etc. etc. Everyone thinks, "Oh, I never would forget that." But we know from a lot of studies from the past 30 years that people aren't necessarily right. You can't even convince people that their memories are wrong. All you can say is that data would suggest your memory's wrong.

With emotional events like 9/11, I think we do have better memory for the important details [as compared with a neutral event]—we just don't have great memory for all the details. And we think we do, and that's the real contrast. Whereas, if I told you that you don't remember the details of your 26th birthday, you wouldn't be surprised, necessarily.

What exactly did you learn about the accuracy of 9/11 memories in the national survey?
It's important to mention that when we talk about "accuracy" here, we mean accuracy for details like, how did find you find out about 9/11, or who were you with? It's not the case that you don't have a fairly vivid image in your head of the planes crashing into the building. No one's forgetting 9/11 occurred.

Our measure of accuracy is consistency with what people told us in the survey the week after the attack. From that first survey to the second survey a year later, the overall consistency of the details of how they learned of 9/11 was only 63 percent. At the third survey, three years after the attack, consistency was 57 percent. So people were only a little more than 50 percent right for a lot of the details.

But they were particularly bad at remembering what their emotions were after 9/11—accurate only about 40 percent of the time, after a year. And yet overall, for all those details, people's confidence in their memories was, on average, greater than 4 on a scale of 1 to 5.

How well did people remember the facts about the attack itself?
They were better at memories for the facts of the attack than they were for their personal recollections. For instance, in recalling the number of planes involved, they were 94 percent accurate immediately afterward in the first survey, 86 percent accurate a year later, and 81 percent accurate three years later. But of course, those facts are supported by external reminders from media coverage.

An explicit example is that we also asked, "Where was President Bush when the attack happened?" Eighty-seven percent of people were accurate immediately after 9/11—but they dropped off to 57 percent and then went back up to 81 percent between the second and third surveys. We know that was due to a lot of people seeing or hearing about Michael Moore's film, Fahrenheit 9/11, which went over all the details that occurred.

Are there certain aspects of our personal recollections of 9/11 that look more reliable than others?
My research group at NYU has started to look at what kind of details are people better at and worse at in their emotional memories. Are there some details that are actually fine, that go along with the enhanced confidence you have in memory for details. And in a recent laboratory study—the paper is under review—we found that with memories for when and where, confidence does predict accuracy for emotional events, as well as neutral events. With when here, I mean the order of when one event happened relative to another event. We then went back to the 9/11 survey data, and we did see that memory was better for where than the other details. In recalling where they were when they learned of the attack, people were 89 percent accurate at survey two and 83 percent accurate at survey three.

Does it really matter if we don't have all the contextual details right in these emotional memories?
No. Again, I think we actually get better memory for the important details, but just worse memory for the other details. It's important to remember that the 9/11 attack occurred, right? I think—and it's just a theory—that part of why we have this super-enhanced feeling of remembering and sense of vividness for these highly emotional events is that knowing that they occurred matters a lot. Knowing the details about how they occurred, who told you about it, doesn't necessarily matter.

But the enhanced confidence that we have in the memory lets us rely on it and act quickly. You want to be confident of that previous memory, the main point of which was that there was this terrorist attack that could threaten my life and now I have to be wary of things that might be similar. Because you don't want to have to go, "Gee, does this remind me of anything...?" before you act on those types of threats in the future.